Polytetrafluoroethylene (PTFE) is a versatile and useful material. In the form of a powdered resin, PTFE can be molded in sheets and other shapes, or directly into a finished part or product. Several types of molding processes useful with PTFE have been developed. Compression molding involves placing a layer of PTFE resin under sufficient pressure to create a preform that can be sintered into a finished shape, which is usually then coined. Isostatic molding involves filling the void between a rigid molding surface and a flexible bladder with PTFE resin. The bladder is then pressurized, and squeezes the resin between the bladder and the molding surface. The bladder is then removed and the resin is sintered. Isostatic molding is expensive and cumbersome, but is well-suited for forming complex and intricate shapes such as valve linings. In fact, the body of the valve itself can be used as the molding surface, thus permitting the valve lining to be formed in situ without requiring secondary shaping or cutting operations. However, for many applications, compression molding remains the simplest and most economic method of producing PTFE articles.
Sheets, bars or other shapes of compression molded PTFE are used to create many useful articles that take advantage of its low coefficient of friction and chemically impervious nature. However, the structural characteristics of a shape formed from virgin PTFE are less than ideal for many situations. It is known, for example, that virgin PTFE exhibits a large degree of cold flow (i.e., "creep") when exposed to continuous loads at elevated temperatures. For example, at 500 psi and 73.degree. F., virgin PTFE will cold flow from an initial deformation of 0.9% to about 1.8% in 10,000 hours. At 212.degree. F., an initial deformation of 3.5% would increase to approximately 7% in the same time period. It is also known that PTFE exhibits deformation due to compressive stress. At stress levels of 500 psi and at a temperature 73.degree. F. , this strain is almost negligible, while at 300.degree. F. the strain at this loading is about 5%. A similar strain level, 5%, is exhibited at 73.degree. F. when the compressive stress is raised to about 1,900 psi. Thus, in applications where the PTFE component is under a compressive load and/or exposed to elevated temperatures, cold flow and deformation due to compressive stress are important considerations.
Nevertheless, it is known that the problems of cold flow can in some instances be overcome by adding fillers to virgin PTFE resins. The use of filler materials is well known in the art of forming shapes from PTFE. See "Selecting the Right `Teflon` TFE Compound For Best Performance," Journal of "Teflon" Volume 13, No. 2 (1972). Examples of filler materials include glass fibers, carbon, graphite, bronze and molybendum disulfide (MoS.sub.2). The modification of the performance of PTFE resins by the use of fillers affects certain mechanical properties and permits resin filler compositions to be tailored to the electrical and mechanical requirements of the application. In general, PTFE resins can be compounded with fillers to increase strength characteristics such as stiffness and resistance to cold flow, to increase thermal conductivity, and to increase hardness and resistance to wear. However, it is also known in the art that any filler raises the low coefficient of friction exhibited by parts formed from virgin PTFE resins and that the increases in hardness and resistance to wear result in an article that abrades or wears cooperating surfaces.
Modified PTFE resins are currently available that provide products exhibiting improved resistance to cold flow without sacrificing the low coefficient of friction of virgin PTFE. Additionally, products made from these modified resins have been found to exhibit better "weldability" to themselves and other PTFE products when used in compression molding. Examples of such modified PTFE resins are TFM1600 and TFM1700 sold by Hoechst, and TG70J and TG170J sold by dupont. One example of such a resin is disclosed in U.S. Pat. No. 4,408,007--Kuhls et al., which is incorporated herein by reference. In some instances the use of such resins is economically impractical since they cost between about 50% and 180% more than standard virgin PTFE.
Therefore, there remains a long felt, yet unsolved need to provide components formed from PTFE that retain the properties of virgin PTFE such as a low coefficient of friction on sliding surfaces, that also exhibit acceptable resistance to cold flow or have other improved properties usually provided by filled and/or modified resins, and that provide this combination of properties in an economically viable form.
Composite PTFE articles are known that are comprised of different types of PTFE. U.S. Pat. No. 5,032,335--Wilson discloses sealing elements sliced from a cylindrical billet comprised of two types of PTFE that are sintered together. The cylindrical billet is formed by arranging two separately formed, unsintered tubular billets concentrically within one another then adding pressure to the combined part and sintering the resulting composite billet. One of the unsintered tubular billets is comprised of filled PTFE while the other uses virgin resin. The separately formed, unsintered billets are compacted using a pressure between 250 to 2,500 psi. After assembly, an axial compressire pressure of between 2,000 to 25,000 psi is applied to fully compress unsintered billets into a preform, which is then sintered at 650.degree. F. to 750.degree. F. for two to 48 hours. A "sandwich" PTFE structure consisting of a layer of unfilled PTFE between two filled layers is disclosed in U.S. Pat. Nos. 4,961,891 and 4,900,629, both to Pitolaj. The disclosed composite structure is said to be highly compressible and is made by fusing sheets of PTFE together using calendar rolls. The resulting laminate is then sintered at 650.degree. F. A multi-layer PTFE seal is also disclosed in U.S. Pat. No. 4,147,824--Dettmann et al. The disclosed seal comprises an outer layer and a porous layer made by washing a filler material from the PTFE. The first layer is fully compressed with a pressure of 300 bar (4,410 psi) and a second, filled layer is added and compressed with the same pressure. The resulting preform is then sintered and the filler material washed out, leaving a porous layer.
Isostatic molding can also be used to create composite structures from homogeneous preforms of different types of PTFE to achieve a unitary non-homogeneous structure, as shown in U.S. Pat. Nos. 4,267,237 and 4,102,966 both to Duperray et al. The preforms must be compressed by a pressure of at least 10 bar (147 psi) since compression using lesser pressures will create preforms that are too brittle to be handled. The preforms are placed in an isostatic mold and further compressed to fuse them together or fuse them with a layer of powdered resin using isostatic pressures between 100 and 1,000 bar (1,470 and 14,700 psi).
However, it has been found that processes by which fully sintered sheets of two types of PTFE are laminated together are likely to delaminate, particularly under severe conditions of temperature and pressure. Conversely forming a preform from two layers of powdered resin that are simultaneously compressed will result in undesirable mixing and contamination of the separate "layers" of different types of PTFE.
One example of an application where the current state of the art fails to provide an adequate material is for the pressure pads used to as sliding surfaces for the belts that compress sheets of particleboard as part of the manufacturing process performed by the Bison Hydrodyn press, manufactured by Bison Werke, Springe, Germany. The construction and operation of this press and similar types of manufacturing equipment is well known to those of ordinary skill and is shown, for example, in U.S. Pat. No. 4,850,848--Greten et al., which is incorporated herein by reference. As shown in FIG. 1, the Bison press and similar machines apply heat and pressure simultaneously to the components of the finished particleboard throughout the length of the machine. The heat is applied by hot lubricant on to the back side of stainless steel bands. The lubricant is pumped through PTFE pressure pads and provides a layer of lubricant between the pads and the bands, in addition to acting as a heat transfer medium. The PTFE pressure pads are attached to the platens that transfer vertical compression force provided by hydraulic cylinders. As seen In FIGS. 2-3, the pressure pads 52 are held in removable carrier trays 50 that form part of the press platens. The carrier trays 50 have angled slots 54 to retain the lateral edges of the PTFE pads 52 while retaining plates 56 are affixed to the perimeter of the carrier trays 50 to secure the pressure pads 52 in place. The lubricating fluid flows through passages 53 in the pads 52 and lubricates the bands.
In practice, it has been discovered that the pressure pads 52 in the above-described system fail in service by a mode referred to as "smearing." Smearing is the migration and thinning of the pressure pads without any wear, in other words, there is no appreciable weight difference between a new pad and a failed pad. Theoretically, the stainless steel belts ride on a layer of oil, the heating oil, that is pumped through the pads between the back surface of the stainless steel belts and the pads. Although the layer of oil is relied upon to prevent the pad from scraping and wearing against the stainless steel belts, in use, the hot oil system sometimes fails and the low friction coefficient of friction between the PTFE and the stainless steel belts is helpful. Moreover, the softer PTFE pads will wear before the stainless steel belts. For these reasons, PTFE is the preferred pressure pad material. However, because the operating temperatures (360.degree.-390.degree. F.) and pressures (300-500 psi) are relatively high, the PTFE exhibits a significant degree of cold flow, ultimately leading to the "smeared" failure mode described above.
Therefore, in this type of equipment and many others, it would be desirable to provide a PTFE component such as the pressure pads that has an exposed sliding surface with a low coefficient of friction, yet resists cold flow at elevated temperatures and pressures. It be further desirable to provide such a component on an economically justifiable basis. It is therefore an object of the present invention to provide improved methods of compression molding PTFE resin into sheets, structural shapes or component parts. It is generally an object of the present invention to provide methods whereby filled and virgin PTFE resins may be compression molded into a unitary non-homogeneous composite structure exhibiting two or more distinct layers having different properties. It is a further object of the invention to permit filled and virgin PTFE resins to be molded together so that the virgin PTFE forms a sliding surface while the remainder of the material is comprised of a filled resin, wherein the filler is chosen to reduce cold flow. It is also an object of this invention to provide methods of manufacture and articles made thereby that make economic use of the various types and grades of PTFE resin, including modified PTFE.